This invention relates to a powder supply element particularly to an element for supplying polycrystalline silicon powder continuously into a crucible for use in a single crystal pulling apparatus.
Silicon single crystals are made by the conventional Czochralski method (CZ method). According to this method, a crucible is provided within a vacuum melting furnace in a single crystal pulling apparatus. Polycrystalline silicon in the crucible is heated into molten silicon by an annular heater which surrounds the crucible With pulling of a single crystal, polycrystalline silicon powder is supplied into the crucible so as to maintain a fixed height of the liquid surface of the molten silicon and to maintain the impurity concentration of the molten silicon constant (For example, the apparatus as described in Japanese Patent Application No. 103918/86 filed on May 8, 1986 may be used).
However, since the silicon polycrystal powder to be supplied has a particle size ranging from about 100 microns to about 3 mm, part of the fine particles after being ejected from a supplying pipe will be suspend in the air and reach the vicinity of the solid-liquid interface of the single crystal without falling directly onto the liquid surface of the molten silicon in the crucible. As a result, the uniformity of the crystal being grown deteriorates. Further, some part of coarse particles of the powder will reach the vicinity of the solid-liquid interface in a solid state without melting promptly.
The heat generated by the heater is transmitted from the side peripheral portion of the crucible and heat convection takes place in the molten crystal component. The current directions of the heat convection are an upward direction near the side peripheral portion of the crucible, an inward direction near the liquid surface of the molten crystal component, a downward direction near the central axis of the crucible and an outward direction near the bottom of the crucible, respectively, whereby the heat transmitted from the heater to the molten crystal component through the side peripheral portion of the crucible can be quickly transmitted toward the center of the crucible. Therefore, a temperature at the liquid surface of the molten crystal component is near a temperature at a solid-liquid interface of the single crystal in the crucible. Thus, the polycrystalline component powder cannot be quickly melted. Non-melted powder is apt to be mixed with the single crystal while it is being pulled so that the quality of the single crystal decreases.